Peroneal tendon instability can be divided into intra-sheath, where abnormal tendon movement occurs without injury to the superior peroneal retinaculum (SPR), and extra-sheath, where the retinaculum is compromised. Each category includes important subtypes that must be recognized for accurate diagnosis. Frequently underdiagnosed, it is a common cause of chronic lateral ankle pain and should be suspected in patients with a history of dorsiflexion or inversion injuries. Early recognition is critical to avoid mismanagement, as it is often mistaken for lateral ligament injuries.

Dynamic US enables real-time visualisation of tendon subluxation during stress manoeuvres, offering significant advantages over static imaging modalities such as magnetic resonance imaging (MRI). Early diagnosis allows appropriate therapeutic planning, preventing long-term complications. Treatment options range from conservative measures to surgical interventions, depending on the severity of instability and the patient’s functional demands.